Senescent cells as a source of pro-geronic factors several classic experiments using heterochronic parabiosis - surgically joining the circulatory systems of young and old animals - indicate the existence of at least two classes of circulating factors that influence aging. One class is abundant in young animals, declines with age, and can rejuvenate tissue homeostasis when given to old animals (anti-geronic factors). A second class is scarce in young animals, increases with age and can reduce tissue homeostasis when supplied to young animals (pro-geronic factors). Pro-geronic factors have attracted less attention than anti-geronic factors, yet are equally critical targets for therapies aimed at rejuvenating the aged systemic milieu. This proposal focuses on circulating pro-geronic factors secreted by senescent cells. Cellular senescence is a stress response that results in an arrest of cell proliferation and the secretion of many inflammatory cytokines, chemokines, growth factors and proteases, termed the senescence- associated secretory phenotype (SASP). Senescent cells increase with age in many vertebrate tissues, but the reason for this increase is not clear. Recent findings show that senescent cells can confer senescent phenotypes on neighboring cells, and our preliminary data suggest one or more circulating factor is responsible for spreading senescent phenotypes to non-senescent cells in vivo. We propose to study this secondary senescence and its role in creating an aged systemic milieu. Two laboratories will join forces to use heterochronic parabiosis and a novel transgenic mouse model to determine the kinetics, extent and cell type or tissue specificity of the ability of factors in the circulatin of aged mice to induce a senescence response in cells in young mice. We will phenotype secondary senescent cells induced by circulating factors present in old animals with regard to cell type and senescence markers, including the SASP. We will also test the idea that parabiosis-induced secondary senescence and increased circulating HMGB1 promote aging phenotypes in several tissues. In parallel, we will characterize the factors produced by secondary senescent cells to identify possible additional pro-geronic proteins. Finally, we will interfere with circulating HMGB1 activity to test the idea that interference can blunt the ability f the old circulation to induce secondary senescence and aging phenotypes. These experiments will provide much needed insights into pro-geronic factors, the neutralization of which will be essential for strategies aimed at rejuvenating the systemic milieu.